<!DOCTYPE html>
<html>

<head>
    <meta charset="utf-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <title>Page Title</title>
    <meta name="viewport" content="width=device-width, initial-scale=1">
</head>

<body>
    <input type="text" name="n1" id="n1">+
    <input type="text" name="n2" id="n2">
    <button type="button" id="btn">=</button>
    <input type="text" name="result" id="result"><br>
</body>

</html>